1. Field of the Invention
The present invention relates to a lens barrel and an image pickup apparatus and, more particularly, to a collapsible lens barrel and an image pickup apparatus having the lens barrel.
2. Description of the Related Art
As a lens barrel mounted in an image pickup apparatus such as a film camera using silver halide film or a digital camera using an image pickup element, a collapsible lens barrel having an advancing cam mechanism is known.
The collapsible lens barrel is housed in a casing of an image pickup apparatus by being retracted along an optical axis of an image pickup optical system when photographing is not performed, i.e., when the power supply for the image pickup apparatus is off, and is projected toward a subject by being extended along the optical axis when photographing is performed, i.e., when the power supply for the image pickup apparatus is on. According to such an arrangement, the size of the lens barrel can be reduced and the zoom ratio can be easily changed through adjustment of the focal length of the image pickup optical system.
In recent years, there has been a demand for further reducing the size of extendable lens barrels. In general, reduction of the size of an extendable lens barrel causes the zoom ratio variable function to degrade, since the amounts of movement of lens groups constituting an image pickup optical system are reduced.
The zoom ratio variable function is useful in photography, for example, in that it enables changing the angle of view as a photographable region. Therefore the development of techniques relating to internal mechanisms of extendable lens barrels is being pursued to achieve a further reduction in size of extendable lens barrels without degrading the zoom ratio variable function.
For example, a technique to reduce the size of a cam ring while maintaining a desired movable distance of a lens group in such a manner that a second lens barrel is transferred from one cam groove provided in a moving cam ring to another cam groove provided in the moving cam ring is known (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2004-85932).
A technique to secure the desired movable distance of a lens group in a mechanism for a zoom lens barrel by setting the movable distance of the lens group to the sum of a movable distance by drive of a second drive member and a movable distance of the second drive member by a first drive member is also known (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2007-219435).
In the technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-85932, however, play is positively provided between three cam followers of the second lens barrel and three cam grooves engaging with the cam followers in order to smoothly perform transfer between the cam grooves. This structure entails a problem in that the accuracy with respect to eccentricity for example deteriorates and a reduction in drive accuracy results.
In the technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2007-219435, a cam rotation transmission mechanism is complicated and the construction of a mechanism for driving the second drive member is also complicated. It is, therefore, difficult to achieve a reduction in size of the lens barrel while maintaining the desired zoom ratio variable function. There is also a possibility of a reduction in the drive accuracy because the drive accuracy can be largely influenced by parts accuracies.